The optical-transceiver is a circuit module incorporated into an optical transponder, and performs bidirectional optical communication by using an optical fiber by performing electrical-optical conversion of an electric signal. A transmission and reception operation of information using an optical-transceiver will be explained with reference to FIG. 4.
FIG. 4 is a block configuration diagram illustrating a related line card. The line card is disposed between a line (network) and a client, and includes an interface of information input and output. In FIG. 4, the line card is constituted by a framer 100 and an optical-transceiver 110.
In a case where information is transmitted from the client side to the line side, the framer 100 applies multiplexing process for the line side transmission to an electric signal 121 received from the client, and structures a frame. Thereafter, the framer 100 converts it into a transmission format (for example, SFI: Serdes Framer Interface) between the framer 100 and the optical-transceiver 110, and outputs the electric signal 122 to the optical-transceiver 110.
In this case, the electric signal 121 from the client side is a signal that is, e.g., multiplexed for an optical signal according to, for example, Synchronous Transport Module (STM) in Synchronous Digital Hierarchy (SDH) specification, and the electric signal 121 has an optical channel transmission speed and the like defined by Optical channel Transport Unit (OTU). It should be noted that the electric signal 121 may be a signal based on other specifications such as Gigabit Ether.
The optical-transceiver 110 converts the input electric signal 122 into an optical signal in a format for line side transmission, and outputs the main signal 123 optically modulated according to the format to the line side.
On the other hand, in a case where information is transmitted from a network structured by an optical fiber and the like to the client side, the processing opposite to the above processing is performed. More specifically, the optical-transceiver 110 receives the main signal 124, i.e., the optical signal, from the line side, converts it into an electric signal, and outputs the electric signal 125 to the framer 100. The framer 100 performs signal processing such as frame conversion on the input electric signal 125, and thereafter outputs an electric signal 126 including information about the main signal 124 to the client side.
In this case, in a case where an abnormality such as communication interruption occurs in an optical fiber at the line side, the optical-transceiver 110 is unable to receive the main signal 124, and is unable to normally generate the electric signal 125 which is to be output to the framer 100. In this case, depending on an LSI (Large Scale Integration) constituting the framer 100, the electric signal 126 which is output to the client side is stopped intermittently or completely.
PTL 1 discloses an optical transmission apparatus transmitting an AIS (Alarm Indication Signal) signal indicating an occurrence of a malfunction in a case where a frequency skip of an input signal occurs due to communication interruption at the line side. The optical transmission apparatus of PTL 1 is constituted by an optical transmission and reception unit including a detection unit for detecting communication interruption at the line side and an electric signal processing unit. Further, the optical transmission apparatus includes a network side transmission PLL (Phase Locked Loop) unit including a voltage control oscillation unit, a clock switch control unit, a selector, and a frequency dividing unit.
In a case where a communication interruption occurs at the line side, the optical transmission apparatus is configured such that a detection unit of the optical transmission and reception unit detects the communication interruption, and accordingly, the clock switch control unit controls the selector to switch the connection destination from the electric signal processing unit to the output side of the network side transmission PLL unit. More specifically, the output of the voltage control oscillation unit of the network side transmission PLL unit is switched to a clock obtained by dividing the frequency by four. A frequency dividing unit disposed at a stage subsequent to the selector divides the frequency by 255. As a result, the amount of variation in the phase before and after the switching is compressed to 1/255. Therefore, the electric signal processing unit using the frequency in which a frequency skip, i.e., the amount of variation in the phase, is suppressed can transmit a stable AIS signal.